I think the 5R55S transmission is like the 5R55W on my '02 Explorer - takes Mercon V and has no dipstick. It is filled through an inner small plug in the drain pan plug with a special adapter. PITA for the home mechanic like me, but as you are doing it in a shop, less hassle.
Ford Motorcraft Mercon V fluid is a synthetic blend, IIRC. You could certainly just load in fresh Motorcraft Mercon V. For synthetic fluids you have a choice:
1) Mobil 1 Synthetic ATF. This is a good fluid and the one I used for a recent partial fluid change.
2) Redline D4. A great choice for a Mercon V compatible fluid - less widely available than the Mobil 1 and more expensive.
3) Amsoil ATF. This fluid has a lower 100C cSt Viscosity than a Mercon V fluid (6.8 vs. 7.4) so I am concerned that it is less compatible with a Mercon V tranny than the other choices.
4) Valvoline Full Synthetic Mercon V. An officially licensed Mercon V full synthetic ATF. I've never seen it on shelves so it may only be special order item.
5) Royal Purple Max synthetic ATF. I do not know much about this fluid.
Next spring I am planning to drop the transmission an, change the filter and do a complete fluid replacement. Right now I am planning to use the Redline D4 ATF as I think it may be a bit higher quality than some other the other fluids and is a better match for Mercon V than the Amsoil ATF, at least based on viscosity.
Rumple